This PRAM 1 Study will be a comparative study of the following three treatment arms: ZDV + D4T + Ritonavir vs. ZDV + 3TC + Ritonavir. Subjects enrolling in PRAM 1 must be between 24 months and 17 years of age. Children under 24 months will not be enrolled due to the lack of Phase I safety data available on the combinations of D4T + Ritonavir and ZDV + 3TC + Ritonavir.